A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems also provide seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs).
Discrete Fourier Transform (DFT) spread orthogonal frequency division multiplex (OFDM) is an uplink transmission scheme of LTE. The DFTS-OFDM-based LTE uplink transmission scheme allows for uplink transmissions received from different UEs within a cell do not cause interference to each other. To achieve this intra-cell orthogonality, it requires uplink alignment from different UEs. LTE includes timing advance mechanism to ensure the uplink alignment. When a UE needs to establish a RRC connection with an eNB, the UE transmits a Random Access Preamble. Upon receiving it, the eNB estimates the transmission timing of the UE. The network controls the uplink alignment by responding with a Random Access Response that consists of timing advance command (TAC). The timing advance compensates for the propagation delay between the eNB and the UE and varies with time, due to UE mobility. During TA maintenance phase, the eNB measures the timing of the received UL data and adjusts the UL timing by TA command. The UE tracks the validity of its UL timing by means of a timing alignment timer (TAT). The network determines the timing-advance value for each UE.
Carrier aggregation (CA) is introduced to improve system throughput. With carrier aggregation, the LTE-Advance system can support peak target data rates in excess of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Such technology is attractive because it allows operators to aggregate several smaller contiguous or non-continuous component carriers (CC) to provide a larger system bandwidth, and provides backward compatibility by allowing legacy users to access the system by using one of the component carriers.
With Carrier aggregation, a single UE may be assigned radio resources on more than one CC. In some cases, multiple CCs share the same timing advance value and belong to the same timing advance group. In other cases, multiple CCs have different timing advance values and belong to different timing advance group (TAG). This is because the DL receptions of different CCs are from different propagation paths. If the time difference between the different paths is larger than a threshold, then the delay becomes non-negligible. As a result, multiple timing advance groups are required such that different timing advance values are applied to different CCs to avoid inter-symbol interference. In one example, the need for different timing advance may arise due to inter-band carrier aggregation, or when transmission for one band is routed via a frequency selective repeater while transmission for another band is not. In another example, DL signals of different bands are routed through different source nodes, such as remote radio heads (RRH) located some distance apart.
Inter-eNB carrier aggregation is introduced in the recent development of LTE. Inter-eNB carrier aggregation is configured to enable the UE to perform normal data transmission and reception through multiple serving cells originated from different eNBs or different sets of co-located antennas. LTE introduces small cell network. The small cell network includes small eNBs with low transmission power and simplified protocol stacks/functionalities together with the normal eNBs. The small cell architecture enhances data throughput and reduces the mobility signaling overhead. In an anchor-based small cell network, a UE is housed in an eNB, which is referred to as an anchor eNB of the UE. UE anchor is UE specific, a UE anchor is a point where the Core Network connection of the UE is terminated, that does not have to be relocated when the UE moves in a local area covered by cells of multiple base-stations. UE serving cell(s) can be controlled by an eNB that is different from the anchor eNB, which is referred to as a drift eNB of the UE. When the UE is served by both anchor eNB and drift eNB, the control of the UE and the user plane functionality is split between the anchor eNB and the drift eNB.
Inter-eNB carrier aggregation requires new approaches for uplink alignment, monitoring and management. When a UE is configured with inter-eNB carrier aggregation, the UE performs normal data transmission/reception through multiple serving cells originating from different eNBs or different set of co-located antennas. Although these eNBs provide services together to the UE at the same time, the same eNBs may serve multiple UEs at the same time. Therefore, each eNB has to guarantee that the uplink transmissions from different UEs are time-aligned. This is different from the existing intra-eNB carrier aggregation when only one eNB is responsible for the uplink channel and timing-advance value to the UE. Further, for inter-eNB carrier aggregation, UE needs to handle TACs coming from different eNBs or different sets of antennas. Inter-eNB carrier aggregation also provides advantages such that when one eNB is out of synchronization, the network can select a different eNB to continue provide data transmissions. Therefore, unlike intra-cell carrier aggregation, when one TAG is out of synchronization, the UE can continue its data transmission through another eNB.
Inter-eNB carrier aggregation requires enhanced mechanisms for uplink alignment, monitoring and management. The current invention provides enhanced mechanisms to handle uplink alignment, monitoring and management procedures for inter-eNB carrier aggregation.